This disclosure relates generally to the field of naturally emitted gamma ray spectroscopy of subsurface formations from within a wellbore. More particularly, the invention relates to techniques for gain stabilization of scintillation-type radiation detectors used for detecting gamma radiation and characterizing its energy without the need for a radioactive reference source to he used in connection with the scintillation-type radiation detector.
Measurement of geological formation content of thorium, uranium, and potassium provides valuable data for analysis of the geophysical character of underground formations. This information is of interest in the oil and gas exploration industry. To measure the concentration of these elements a well logging instrument containing a gamma ray spectrometer may be lowered into a borehole, for example, by either wireline cable or drill pipe. Because the gamma ray spectrometer energy response calibration may drift, over time and temperature during use thereof in a borehole, a means to maintain the energy response calibration is required. It is known in the art to use a low-activity radioactive source installed in the gamma ray detector to provide a stable energy reference peak that the well logging tool hardware or software can use to correct for any drift in energy response calibration. Am241 is typically used as a reference source.
While using an Am241 reference source is a fairly straightforward solution of the energy response gain stabilization problem, it may have disadvantages. One is that the reference energy peak for an Am241 source is 59.5 KeV, but the energy range of the typical spectrometer is from a few hundred Kev to more than 2.6 Mev. This results in a dynamic range of nearly 45 from the stabilization peak energy to the upper end of the spectrometer operating range. This large dynamic range can make stabilization difficult. Another disadvantage to using an Am241 reference source is it may require licensing and special handling aid storage procedures because it is a chemical isotopic radiation source. This increases the administrative costs associated with such type of well logging tool. An alternative to the use of Am241 is described in U.S. Pat. No. 5,023,449 issued to Holenka et al., wherein a higher energy peak but extremely low activity Na22 source is used in coincidence mode between two detectors to stabilize a pair of detectors. The foregoing described technique may overcome some of the disadvantages of using an Am241 source, but requires two detectors located very close together.